Polymer composites called “nano-composites” are generally prepared by compounding an organic polymer such as polyamide, polystyrene, polypropylene, polyimide or polyurethane with clay. Such polymer composites have been reported to exhibit improved properties such as elastic modulus, heat deflection temperature, gas permeability and combustion rate, due to a layer of clay having a large aspect ratio finely dispersed therein (For example, see Non-patent Document 1).
It is preferable that clay minerals are present in a great content in the polymer composites in view of performance improvement. However, it is also important to efficiently accomplish the desired properties with a lower content of clay minerals. Research to date commonly utilizes polymer composites comprising 0.2 to 5% by weight of inorganic compounds and does not utilize polymer composites comprising 0.1% by weight or less, or 10% by weight or more of inorganic compounds. This is the reason that performance improvement becomes negligible if the content of inorganic compounds used is too low, while nano-scaled fine and uniform dispersion of clay minerals in the obtained composites cannot be accomplished due to a large increase of viscosity in the preparation process, or the composites become fragile and mechanical properties (strength or elongation) thereof are thus deteriorated, if the content of inorganic compounds used is too high.
In an attempt to solve such problems, several conventional methods have been suggested. For example, as a nano-composite material with superior mechanical properties, an organic-inorganic composite hydrogel in which a clay mineral is dispersed uniformly in an organic polymer in a wide range of clay mineral content has been disclosed. It has been disclosed that, by polymerizing acrylamide or methacrylamide derivative, (meth)acrylic acid ester or others in the presence of a water-swellable clay mineral and a polymerization initiator in an aqueous medium, a polymer composite with superior mechanical properties is prepared (for examples, see Patent Documents 1 and 2).
Also, as a nanocomposite material exhibiting superior mechanical properties in a dry state, a polymer composite in which a polymer obtained from a water-soluble (meth)acrylic acid ester and a water-swellable clay mineral form a three-dimensional network has been disclosed. This polymer composite may be prepared by dissolving or uniformly dispersing a water-swellable clay mineral, a water-soluble (meth)acrylic acid ester and a polymerization initiator, and optionally a catalyst and/or an organic cross-linking agent, in water or a mixed solvent of water and an organic solvent, polymerizing the water-soluble (meth)acrylic acid ester, and drying the resulting polymer to remove the solvent (for example, see Patent Document 3).
Also, a method for rapidly preparing an organic-inorganic composite hydrogel while being not susceptible to oxygen has been disclosed. In accordance with this method, an organic-inorganic composite hydrogel with superior mechanical properties can be prepared by reacting a water-soluble acryl-based monomer in the presence of a water-swellable clay mineral by irradiating with an energy beam in a reaction solution in which a water-insoluble polymerization initiator is dispersed in an aqueous medium (for example, see Patent Document 4).
All of the aforementioned organic-inorganic composite hydrogels and polymer composites are bulk bodies and are prepared via gelling of the overall reaction system.
Meanwhile, in the field of biochemistry or medicine and industries such as the automotive industry, there is a need for organic-inorganic composite dispersions (coating materials) which exhibit superior film formability and enable formation of films exhibiting superior adhesion to substrates, or provide functionalities such as cell culture performance and antifogging properties. However, the aforementioned patent documents do not disclose an organic-inorganic composite dispersion in which organic-inorganic composite particles are dispersed in an aqueous medium, which satisfy these properties and a method for preparing the same.
Meanwhile, plastic (for example, polystyrene) vessels have been used for cell (e.g., animal tissues) culture substrates. The surface of these vessels is treated with plasma or is coated with silicon or cell adhesion agents in order to enable efficient cell culture. In the case where these cell culture vessels are used as culture substrates, the cultured (proliferated) cells are adhered to the surface of the vessels, which requires use of proteases such as trypsin or chemicals in order to detach and collect the cells. The operation for detaching the cells using enzymes or chemicals is complicated and has the risk of incorporation of various germs or impurities such as DNA or RNA. In addition, disadvantageously, regions in which cells are linked to substrates or linkages between cells are cleaved, and the cells cannot be thus collected in their proliferated forms such as sheet forms, or natures thereof are changed.
Recent research has reported use of a substrate in which a polymer (e.g., poly(N-isopropylacrylamide)) having a lower critical solution temperature is considerably thinly coated on the surface of a cell culture vessel. The polymer is hydrophobic at a cell culture temperature and cells are thus adhered to the polymer. After cell culturing, the polymer is treated at a low temperature and thus becomes hydrophilic. As a result, the adhesion between the cells and the polymer is deteriorated, and cells can thus be detached in sheet form from the substrate without using hydrolases or chemicals (for example, see Patent Documents 5 and 6, non-Patent Document 2).
However, polymers such as poly(N-isopropylacrylamide) exhibit poor adhesion to the surface of plastics such as polystyrene and applied layers thereof may be readily detached upon exposure to water. In order to prevent detachment of polymer layers from the plastic surface upon exposure to water, the polymers should be fixed to the plastic surface via a specific means. One fixing method is to apply an N-isopropylacrylamide (monomer) solution to the surface of cell culture substrates and graft-polymerize it via electron-beam irradiation (for example, see Patent Document 7).
The graft-polymerization using electron-beam irradiation necessarily entails cross-linking between polymers and great deterioration in temperature response rate of polymers with the process of cross-linking. Making the polymer hydrophilic disadvantageously involves a long low-temperature maintenance period and damage to the cells due to exposure to the low temperature for a long time. Also, cell culture substrates prepared by this method exhibit greatly deteriorated temperature response of polymers and loses cell detachability, when sterilized by radiation (for example, γ-rays).
Meanwhile, a cell culture substrate which comprises polymeric hydrogel obtained by polymerizing a water-soluble organic monomer in the presence of a water-swellable clay mineral uniformly dispersed in water by irradiation, and have a three-dimensional network structure composed of a polymer of a water-soluble organic monomer (a polymer having a lower critical solution temperature such as poly(N-isopropylacrylamide)) and a water-swellable clay mineral (for example, see Patent Document 8), are disclosed.
In biochemistry, for cell culture manipulation, there is a need for integration of a cell culture substrate with a vessel such as plastic culture dish. However, the aforementioned prior arts do not provide a specific means of integrated cell culture vessels.
In addition, a cell culture substrate using a polymer hydrogel obtained by co-polymerizing methoxyethylacrylate and N-isopropylacrylamide, in the presence of a water-swellable clay mineral uniformly dispersed in water, has been known (for example, see Patent Document 9).
However, the polymer hydrogel disclosed in this prior art is a bulk body, and is not related to an organic-inorganic composite dispersion in which particles of an organic-inorganic composite with superior film formability are dispersed in an aqueous medium. In addition, in the case where the polymer hydrogel is used as a cell culture substrate, cultured cells can be detached using a pincette, but it is not possible to collect all of the cultured cells by naturally detaching them through temperature variation.